With regard to photosensitive silver halide emulsions for use in the photographic field, emulsions other than silver iodobromide emulsions have only limited application in high speed camera photographic elements. As used in the photographic field, silver iodobromide grains normally contain silver iodide in the silver bromide crystal lattice in an amount not greater than the solubility limit in silver bromide, i.e., an iodide content of up to approximately 40 mol %. The iodide contained in a silver iodobromide emulsion has the following merits (1) and disadvantages (2).
(1) An increase in the latent image formation efficiency, increase in the light absorbance (the intrinsic absorbance of the silver halide), improvement in the adsorption of additives and improvement in the graininess.
(2) Inhibition of development, impairment of chemical sensitization.
To date, there have been a great many studies concerning enhancing the aforementioned merits, and minimizing the disadvantages in the evolution of silver iodobromide photographic materials having useful camera speeds. A particularly important consideration has been in regard to the location (e.g., portion or site) of the silver iodide in the silver halide emulsion grains (referred to as "grains" hereinafter).
The following is disclosed on p. 18 of Photographic Emulsion Chemistry (Duffin, Focal Press, 1966). "An important factor to be considered in the case of iodobromide emulsions is the location of the iodide which may be present mainly at the center of the crystal, distributed throughout the grain or mainly on the outside. The actual location of the iodide is determined by the preparation conditions and will clearly have an influence on the physical and chemical properties of the crystal."
In the single jet method wherein both the iodide and bromide salts are charged in their established concentration in the reaction vessel, and then silver iodobromide grains are formed by introducing an aqueous silver salt solution into the reaction vessel, the silver iodide is first precipitated, and it therefore becomes easier for the silver iodide to collect in the center of the grain. On the other hand, with the double jet method wherein both the iodide and bromide salts are simultaneously introduced into the reaction vessel together with the silver salts, it is possible to control the distribution of the silver iodide within the grain as desired.
For example, it is possible to distribute the silver iodide uniformly over the entire grain, or to form a shell of silver iodide or of silver iodobromide with a high silver iodide content on the outer surface (outside) of the grain by decreasing or stopping the addition of the bromide salts during grain formation while continuing with the addition of the iodide salts. JP-A-58-113927 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a silver halide emulsion containing tabular silver iodobromide in which at least 50% of the total projected surface area is comprised by tabular silver iodobromide grains having a thickness of less than 0.5 .mu.m, a diameter of 0.6 .mu.m or more and an average aspect ratio of 8 or more, and in which the said tabular grains have first and second opposite parallel principal surfaces and a central region extending between the said two principal surfaces, and in which the silver iodide content in an inner portion in a transverse direction of the central region extending between the 2 main surfaces is lower than the iodide content in at least one outer portion of the central region. JP-A-59-99433 discloses a silver halide emulsion containing silver halide grains in which 10% (by number) or more of the silver halide grains present in the silver halide emulsion are tabular silver halides having an aspect ratio of 5 or more and containing silver iodide in the region from the center in the long-axis direction or the short-axis direction of the grain to a portion within 80 mol % with respect to the amount of silver in the entire grain (high internal iodide phase), wherein the average iodide content of the high-internal iodide phase is not less than 5 times the average iodide content of the silver halide present outside the phase, and wherein the silver amount in the high-internal iodide phase is 50 mol % or less of the amount of silver in the entire grain. Furthermore, JP-A-60 -1477277 discloses a silver halide photographic emulsion containing silver halide grains wherein, in silver halide grains having a multilayer structure and an aspect ratio of 5 or less, the difference between the average iodide content of 2 arbitrary layers adjacent within the said grains and respectively having homogeneous iodide distributions is 10 mol % or less, and in which the total silver iodide content of the silver halide grains having this multilayer structure is 10 mol % or less.
JP-A-60-14331 discloses a silver halide photographic emulsion containing silver halide grains with a distinct laminar structure, characterized in that they are composed of a core containing 10 to 45 mol % of silver iodide and a shell containing 5 mol % or less of silver iodide, and in that the average silver iodide content is 7 mol % or more. Furthermore, JP-A-61-245151 discloses a silver halide emulsion characterized in that it has a laminar structure with a plurality of different silver halide contents, the silver iodide content of the outermost shell being 10 mol % or less, a high-silver iodide shell with a silver iodide content at least 6 mol % higher than the abovementioned outermost shell is provided on the inside of the abovementioned outermost shell, and an intermediate shell having an intermediate silver iodide content is provided between the abovementioned outermost shell and the abovementioned high-silver iodide shell. With regard to details further described in the above-noted patents, the amount of silver iodide contained in any one of the grains is modified in terms of its location (in particular, whether the silver iodide is present on the inside or outside of the grain to thereby obtain various photographic characteristics.
On the other hand, Y. T. Tan and R. C. Baetzold have calculated energy states for silver halides and, at the 41st annual meeting of the SPSE, have advanced the theory that the iodide has a tendency to form clusters in silver iodobromide crystal grains. With regard to the silver iodide distribution in the tabular silver iodobromide grains described above, there is a variation in the amount of silver iodide contained in different areas of the grain over an interval of at least 300 to 1,000 .ANG., but a more microscopically non-uniform silver iodide distribution is observed in the silver iodobromide crystals as predicted by Y. T. Tan and R. C. Baetzold.
The silver halide grains having a completely uniform silver iodobromide phase as described in Japanese Patent Application No. 63-7853 (corresponding to U.S. Pat. application Ser. No. 07/298,601 filed Jan. 18, 1989) exhibit a high speed due to this uniformity, but are ineffective in terms of speed when the developing time is shortened. It is possible to determine the microscopic silver iodide distribution and dislocation lines in silver iodobromide grains by means of a cooled transmission electron microscope, and the silver halide grains in which the silver iodide distribution is completely uniform as disclosed herein have not been obtained hitherto.